1. Field of the Invention
The present invention is directed to a method and to a base station system for voice transmission via a radio interface in a digital radio communication system having mobile stations and base stations.
2. Description of the Prior Art
A base station system is part of a digital radio communication system, for example a GSM mobile radio telephone network (Global System for Mobile Communications, as known from J. Biala, "Mobilfunk und intelligenete Netze", VIEWEG Verlag, Braunschweig/Wiesbaden, 1995, particularly pages 57-92).
Mobile communication systems enable the setup of a communication connection to a subscriber among a number of mobile subscribers so that information, particularly voice information, can be transmitted via a radio interface. A number of subscribers on the same carrier frequency of this radio interface can be separated by different time slots in a type of system known as , a time-division multiplex communication system. The time-division multiplex method is also referred to as TDMA (Time Division Multiple Access) method. Further methods for separating the subscribers can be employed at the radio interface in addition to or alternatively to time-division multiplex. A frequency-division multiplex is offered in addition to the time-division multiplex in the GSM mobile radio telephone system.
A mobile communication system has at least one base station system that, for example, contains a base station controller that is connected to a number of base stations. The base stations respectively cover radio "broadcast" areas and each base station makes radio-based resources available for mobile stations in the radio area of that base station. The radio areas of neighboring base stations thereby overlap, so that a handover procedure is possible for a communication connection for a mobile station between two base stations. Each base station thus can manage only a limited supply of radio-based resources. In, for example, the GSM mobile radio telephone system, such radio-based resources are formed by channels that are characterized by their frequency and the time slots.
In a GSM mobile radio telephone system, for example, the base station controller assumes the function of a radio channel switching stage within the base station system, and controls the base stations. In particular, the base station controller realizes the functions of management of radio channels of the base stations and administration and execution of handover procedures.
Voice encoders that generate a data rate that is lower than the 64 kbit/s data rate of PCM30 channels are usually utilized in digital mobile radio telephone systems. This is necessary since the radio-based resources available are tight. For further-development of voice encoders, half-rate encoders that employ half the data rate of the voice transmission have been planned in the GSM and in the D-AMPS mobile radio telephone system. Full-rate and half-rate voice encoders are thus available.
European Application 0 472 511 discloses a digital mobile radio telephone system that employs two different voice encoders. The bit error rate of the transmission between mobile station and base station is thereby regularly measured. The half-rate encoder is employed given a low bit error rate and a full-rate encoder is employed given a high bit error rate.